1. Field of the Invention
The invention relates to an improved system for lubricating the interface between a metallic workpiece and the metalforming tooling used to alter the shape of said workpiece. In particular, the invention relates to an improved system for lubricating that interface in forging operations.
2. Background Art
It is well known in the metalforming arts that some sort of lubrication in the interface between a metallic workpiece and the metalforming tooling used to alter the shape of said workpiece is important for effective metalforming. Such lubricants facilitate the sliding of the workpiece material over the surface of the tooling, which helps the workpiece to conform the shape of the working surfaces of the tooling, and improves the metallurgical structure of the finished workpiece. Lubricants reduce wear on the tooling caused by flow of the workpiece material over the tooling. Lubricants alleviate the sticking of the workpiece to the tooling, thereby facilitating removal of a formed workpiece from the tooling. Lubricants also reduce the likelihood of damaging the surface of the workpiece by scoring, or forming grooves or scratches in the workpiece.
A wide range of lubricants has been employed for various types of metalworking operations. Some of the common lubricants are hydrocarbons and/or chemically modified hydrocarbons ranging in viscosity from light oils to thick greases, polymeric materials, soap-like substances, graphite and similar flake-like substances, and glasses. For convenience in application, these active substances may be used in conjuction with some sort of vehicle or solvent. Various additives such as wetting agents, suspension agents, thickeners or biocides may also be employed. In some cases, a precursor material is applied to the surface of the workpiece or the tooling, and it transforms to another material as a result of thermal decomposition or chemical reaction; the latter material is the actual lubricant. Factors that affect the choice of lubricant include the alloy composition of the workpiece, the physical dimensions of the workpiece, the nature of the metalforming operation, the temperature at which the metalforming operation is to be performed, and cost of the lubricant. Reactivity of the lubricant with either the workpiece material or the forging die must also be considered.
Similarly, a wide range of methods of applying metalforming lubricants has also been employed. In some methods, the lubricant is applied to the workpiece to be reshaped during the metalworking operation. In other methods, the lubricant is applied to working surfaces of the tooling used to effect the changes in shape. Some of the methods employed for applying lubricants are brushing, swabbing, spraying, dipping and electrostatic deposition. Most of these methods are facilitated by dissolving or suspending the lubricant hi some sort of low-viscosity vehicle, which allows the lubricant to be readily spread over the surface to which it is applied. Factors that affect the choice of application method include the choice of lubricant, the physical dimensions of the workpiece, the nature of the metalworking operation, convenience and cost.
In the context of the present invention, a combination of one or more metalforming lubricant(s) with one or more method(s) of application is termed a lubrication system.
It should be noted that there are several different fields of art within the broad area of metalforming arts. Forging, which is the context of the present invention, exists as hot forging, cold forging, isothermal forging, open die forging, closed die forging, press forging and hammer forging, etc. Other common metalforming operations include powder metallurgy, wire and tube drawing extrusion, rolling, heading, coining, sheet forming, and roll forming. By history and custom, the metalforming industry is highly fragmented; that is, firms doing business hi one of these types of metalforming tend to specialize in that one type of metalforming, to the exclusion of the other types of metalforming. Thus, one possessing ordinary skill hi one type of metalforming will not necessarily have ordinary skill in the other metalforming arts. This situation tends to limit the exchange of technical information among the different metalforming arts, and it also tends to limit the extent of obviousness therein.
Some of the teachings in the prior art relating to lubrication systems for forging processes are the following. Huet (U.S. Pat. No. 2,926,138) teaches the application of a paste comprising spherical glass and graphite particles in a vehicle of grease or oil to the forging tooling. In the context of the present invention, the term vehicle is taken to include liquids and liquefiable substances in which solids may be dissolved or suspended. Huet specifically teaches that the glass particles be spherical and comprised of a thermal insulator. Jacobs (U.S. Pat. No. 5,049,289) teaches a range of compositions for lubricants intended for facilitating assembly of threaded pipes and pipe fittings, and the sealing thereof against leakage. These lubricants are somewhat similar in composition to metalforming lubricants, but there is no teaching of the applicability to metalforming. Jain et al (U.S. Pat. Nos. 3,983,042, 4,104,178 and 4,287,073), Feneberger et al (U.S. Pat. No. 4,052,323) and Periard et al (U.S. Pat. No. 5,691,282) teach the use of suspensions of graphite powder in water, to which other ingredients have been added to stabilize the suspension, improve the performance of the lubricant, and for other purposes. Each of these patents teaches application of the lubricants to the die surfaces by spraying. Graham (U.S. Pat. No. 5,495,737) teaches the use of a lubricant mist comprising graghite particles and a vaporizable and polymerizable organic liquid, applied to either heated tooling or a heated workpiece. The prior art cited by Graham refers to application of forging lubricants by spraying, but Graham offers no specific method of application in his teaching.
Lubricants such as those described in the above-cited patents, or commercially available lubricants of generically similar formulations, are routinely employed in metalforming operations, particularly in hot forging of metallic workpieces. In spite of its inefficiency, spraying remains the most common method of application. During the development of the present invention it was estimated that in a typical process for applying a lubricant such as that disclosed by Jain, for each pound of graphite deposited on the die surfaces, roughly 40 pounds of material is discharged into the atmosphere. Some of this material is vaporized liquid carrier; some of it is a mixture of suspension or wetting agents, and the like; some of it is lubricant that is sprayed past the die surfaces. Further, the amount of lubricant employed is typically more than is necessary to provide proper lubrication, for operators of forging equipment often use excess lubricant to ensure that a finished forging does not stick in the die cavity. This approach is not unreasonable, because having a forging stick in the die cavity is, at best, a costly inconvenience in the operation of a forging shop. However, this approach wastes a lot of lubricant, and it leads to deposits of excess lubricant in corners and depressions in the die cavity.
In 1971 Lomax, Jr. was granted a patent (U.S. Pat. No. 3,556,255) for electrostatic deposition of graphite onto the working surfaces of forging dies. However, the apparatus described by Lomax has not been widely accepted in the forging industry. It appears that Lomax's apparatus does not function as well in production as one might expect from the description in his patent. An explanation is found in a more recent patent by Inculet et al (U.S. Pat. No. 5,682,591), who state that "The lubricants that can be electrostatically sprayed in accordance with the present invention ideally have a low electrical conductivity . . . " It would appear that graphite, which is a very effective ingredient in forging lubricants, conducts electricity too well to be effectively applied to forging dies by electrostatic deposition.
The context of the invention by Inculet et al, and that of Fedrigo (U.S. Pat. No. 3,993,979) is applying lubricant to powder metallurgy dies. As metal powders are typically compacted at ambient temperature, the problems are somewhat different than encountered in forging and other metalforming operations. A principal reason for these differences is that powder metal compaction in press dies, the process described by Inculet et al, is typically performed at ambient temperature, where liquid petroleum or stearate lubricants are effective. By contrast, many forging operations are performed at elevated temperatures where such lubricants burn up. Further, the forging and powder metallurgy industries have been generally isolated from each other by the nature of the materials and workpieces, and by the historical development of the two industries. A somewhat similar approach of applying lubricant to a forming die was developed by Staniforth (U.S. Pat. No. 5,017,122) for use in producing pharmaceutical tablets. Inventions by Scholes et al (U.S. Pat. Nos. 4,073,966 and 4,221,185) disclose the use of electrostatic coating to apply lubricants to sheet metals prior to forming those materials in stamping and related manufacturing processes.
The gap between the application of lubricants by electrostatic deposition and the use of electrostatic deposition for applying lubricant to forging dies is the nature of the lubricant. If the lubricant is an organic or metallorganic material having poor electrical conductivity, such as zinc or magnesium stearate, or other waxy organic material, the electrostatic deposition process is effective. These materials are poor conductors of electricity. Graphite, which is particularly effective as a lubricant in hot forging operations, conducts electricity too well.
Many processes for coating a wide range of particulate materials with various coatings are known to those skilled in the art of microencapsulation. This is a mature art, widely used in many industries, such as the manufacture of pharmaceuticals, pesticides, "carbonless" paper and the like. One particular process for microencapsulation, taught by Smith-Johannsen et al (U.S. Pat. No. 3,992,558), employs a fluid energy mill for coating very small particles. Their process is directed toward particles smaller than 20 microns in diameter, and they present an example that describes the coating of particles only 0.03 microns in diameter. Thus, known coating and encapsulation technology includes methods for coating particles ranging from 0.03 microns to more than one millimeter in diameter.